Bird does not prefer older eBGP route - RFC5004 and "older prefer on"
Maria Matejka
maria.matejka at nic.cz
Mon Jul 3 14:24:46 CEST 2023
Well yes, the "prefer older" works only for eBGP, as stated in RFC 5004.
With iBGP, this isn't invoked as you may run into some nasty routing
loops. If you need this feature, feel free to send in a patch.
Maria
On 7/3/23 13:46, Mazur, Dariusz wrote:
>
> Hello Maria,
>
> Thanks for response. Attributes look the same. Maybe the problem is
> these routes are learnt via iBGP, what is not visible in "show route all'
>
> Our simplified topology looks like below:
>
> 1.Host_1 and Host_2 announces 172.232.160.0/19
>
> 2.Host_1 and Host_2 have different ASNs and they use eBGP to peer with
> ToRs
>
> 3.All tors are connected to r01.leaf over iBGP
>
> Host1----eBGP----r01a.tor----iBGP-----r01.leaf
>
> ----eBGP----r01b.tor----iBGP
>
> Host2----eBGP----r02a.tor----iBGP-----r01.leaf
>
> ----eBGP----r02b.tor----iBGP
>
> Show route all from r01.leaf
>
> 172.232.160.0/19 unicast [192.168.196.1__r01a.tor106 2023-06-30] *
> (100) [AS4250627481?]
>
> via 192.168.196.1 on vlan.201
>
> Type: BGP univ
>
> BGP.origin: Incomplete
>
> BGP.as_path: 4250627481
>
> BGP.next_hop: 192.168.196.1
>
> BGP.med: 0
>
> BGP.local_pref: 400
>
> BGP.atomic_aggr:
>
> BGP.aggregator: 23.219.179.225 AS4250627481
>
> BGP.community: (63949,1000) (63949,1002) (63949,1004)
> (63949,1005) (65110,31107) (65310,31107) (65518,31107)
>
> unicast [192.168.196.3__r01b.tor106 2023-06-30]
> (100) [AS4250627481?]
>
> via 192.168.196.3 on vlan.202
>
> Type: BGP univ
>
> BGP.origin: Incomplete
>
> BGP.as_path: 4250627481
>
> BGP.next_hop: 192.168.196.3
>
> BGP.med: 0
>
> BGP.local_pref: 400
>
> BGP.atomic_aggr:
>
> BGP.aggregator: 23.219.179.225 AS4250627481
>
> BGP.community: (63949,1000) (63949,1002) (63949,1004)
> (63949,1005) (65110,31107) (65310,31107) (65518,31107)
>
> unicast [192.168.196.5__r02a.tor106 2023-06-30]
> (100) [AS4250627482?]
>
> via 192.168.196.5 on vlan.203
>
> Type: BGP univ
>
> BGP.origin: Incomplete
>
> BGP.as_path: 4250627482
>
> BGP.next_hop: 192.168.196.5
>
> BGP.med: 0
>
> BGP.local_pref: 400
>
> BGP.atomic_aggr:
>
> BGP.aggregator: 23.219.179.226 AS4250627482
>
> BGP.community: (63949,1000) (63949,1002) (63949,1004)
> (63949,1005) (65110,31107) (65310,31107) (65518,31107)
>
> unicast [192.168.196.7__r02b.tor106 2023-06-30]
> (100) [AS4250627482?
>
> via 192.168.196.7 on vlan.204
>
> Type: BGP univ
>
> BGP.origin: Incomplete
>
> BGP.as_path: 4250627482
>
> BGP.next_hop: 192.168.196.7
>
> BGP.med: 0
>
> BGP.local_pref: 400
>
> BGP.atomic_aggr:
>
> BGP.aggregator: 23.219.179.226 AS4250627482
>
> BGP.community: (63949,1000) (63949,1002) (63949,1004)
> (63949,1005) (65110,31107) (65310,31107) (65518,31107)
>
> Thanks,
>
> Dariusz
>
> *From: *Maria Matejka via Bird-users <bird-users at network.cz>
> *Reply-To: *Maria Matejka <maria.matejka at nic.cz>
> *Date: *Friday, June 30, 2023 at 12:53 PM
> *To: *"bird-users at network.cz" <bird-users at network.cz>
> *Subject: *Re: Bird does not prefer older eBGP route - RFC5004 and
> "older prefer on"
>
> Hello!
>
> I suspect that the routes either aren't all external, or are otherwise
> compared different before it comes to breaking ties. Could you please
> share the `show route all` output to see all the relevant BGP attributes?
>
> Maria
>
> On 6/30/23 11:43, Mazur, Dariusz via Bird-users wrote:
>
> Hello Bird users,
>
> Have you ever use RFC 5004 and “older prefer” knob. I am trying to
> use it but it seems not to work:
>
> *1.Router learns the same route from different ebgp peers, it
> prefers route from r01a and this route is exported to BGP peers*
>
> 172.232.160.0/19 unicast [192.168.196.129__r01a.tor106
> 2023-06-14] * (100) [AS4250627481?]
>
> via 192.168.196.129 on vlan.201
>
> unicast [192.168.196.131__r01b.tor106
> 2023-06-14] (100) [AS4250627481?]
>
> via 192.168.196.131 on vlan.202
>
> unicast [192.168.196.133__r02a.tor106
> 2023-06-14] (100) [AS4250627482?]
>
> via 192.168.196.133 on vlan.203
>
> *2.Once we lose links to r01a and r01b route from r02a is
> preferred and exported to BGP peers. It is expected*
>
> 172.232.160.0/19 unicast [192.168.196.133__r02a.tor106
> 2023-06-14] * (100) [AS4250627482?]
>
> via 192.168.196.133 on vlan.203
>
> *3.When links to r01a and r01b. are again online route, route
> from r01a is pricked as primary and exported to BGP. It causes
> route oscillation*
>
> 172.232.160.0/19 unicast [192.168.196.129__r01a.tor106
> 09:14:19.982] * (100) [AS4250627481?]
>
> via 192.168.196.129 on vlan.201
>
> unicast [192.168.196.131__r01b.tor106
> 09:14:19.896] (100) [AS4250627481?]
>
> via 192.168.196.131 on vlan.202
>
> unicast [192.168.196.133__r02a.tor106
> 2023-06-14] (100) [AS4250627482?]
>
> via 192.168.196.133 on vlan.203
>
> *4.I believe it is default behavior not to prefer older path.
> According to documentation RFC 5004 and "prefer older on" should
> fix my problem, but it does not work.*
>
> Bird doc says:
>
> /prefer older switch/
>
> /Standard route selection algorithm breaks ties by comparing
> router IDs. This changes the behavior to prefer older routes (when
> both are external and from different peer). For details, see RFC
> 5004. Default: off./
>
> *5. According to documentation RFC 5004 and "prefer older on"
> should fix my problem, but it does not work.*
>
> *a) added "prefer older on", bgp flapped and routes were re-learnt*
>
> 172.232.160.0/19 unicast [192.168.196.129__r01a.tor106
> 09:22:12.949] * (100) [AS4250627481?]
>
> via 192.168.196.129 on vlan.201
>
> unicast [192.168.196.131__r01b.tor106
> 09:22:13.527] (100) [AS4250627481?]
>
> via 192.168.196.131 on vlan.202
>
> unicast [192.168.196.133__r02a.tor106
> 09:22:12.683] (100) [AS4250627482?]
>
> via 192.168.196.133 on vlan.203
>
> *b) shut links to r01a and r01b*
>
> *172.232.160.0/19 unicast [192.168.196.133__r02a.tor106
> 09:22:12.683] * (100) [AS4250627482?]*
>
> * via 192.168.196.133 on vlan.203*
>
> *c) unshut links to r01a and r01b, route from r01a is again
> preferred, so looks like "older" knob does not work*
>
> 172.232.160.0/19 unicast [192.168.196.129__r01a.tor106
> 09:27:55.841] * (100) [AS4250627481?]
>
> via 192.168.196.129 on vlan.201
>
> unicast [192.168.196.131__r01b.tor106
> 09:27:54.448] (100) [AS4250627481?]
>
> via 192.168.196.131 on vlan.202
>
> unicast [192.168.196.133__r02a.tor106
> 09:22:12.683] (100) [AS4250627482?]. *---> this route is older*
>
> via 192.168.196.133 on vlan.203
>
> Thanks,
>
> Dariusz
>
> --
> Maria Matejka (she/her) | BIRD Team Leader | CZ.NIC, z.s.p.o.
--
Maria Matejka (she/her) | BIRD Team Leader | CZ.NIC, z.s.p.o.
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